The underground location of water and sewer lines, as well as electrical conduits, makes their replacement difficult. This is particularly so where additional infrastructure has been developed around or on the previously built underground lines or conduits. Often such lines were installed via open trenches years ago and now they cannot be easily re-excavated. Over the years, new developments, such as roads, parking lots, buildings, or landscaping have been placed over the surface of the old lines, thus making re-excavation impossible or unacceptably costly.
Numerous methods have been developed over the years to address the problem of how to replace worn out water and sewer lines without excavating the lines. Typically, such methods will replace the older iron or steel lines with a new line made from a flexible plastic material. One such method, of which this invention is an improvement, is called pipe bursting.
Pipe bursting methods for replacing old, typically metal, water, sewer, or electrical conduit lines make use of a conical shaped mole that is pulled through an existing pipe. The mole is shaped such that it is smaller than the inside diameter of the old pipe at one end of the mole and larger than the inside diameter of the pipe at the other end of the mole, and thus, the mole causes the original pipe to be burst or fractured upon the mole being pulled through the pipe. The original pipe is burst outward radially. Typically, attached to the back of the mole is a length of flexible replacement piping which is drawn into the space formerly occupied by the burst pipe and therefore takes the place of the original. Thus, the new pipe replaces the old pipe without excavating along the entire length of the pipe being replaced. Excavation is required typically only at the ends of the pipe to be replaced.
The basic components of prior art pipe bursting systems include a mole, a length of cable engageable to the mole, a cable pulling device, and a mounting structure for supporting the cable pulling device against an opening through which the mole is to be pulled. In order to pull a mole through an iron pipe, pulling force on the order of 15-75 tons may be required. To provide sufficient pulling force, many prior art pipe bursting processes have used winches of various types. U.S. Pat. No. 5,328,297 to Handford is one example of such a device. However, winches of sufficient size to generate 75 tons of pulling force typically weigh several tons themselves and are frequently mounted on trucks or are attached to other large excavation devices. In some applications, space limitations prevent the use of a winch as the source of pulling force.
Due to the relatively large size of a winch suitable for pulling a mole through an iron or steel pipe, efforts have been made to find smaller devices capable of generating the necessary pulling force. These devices typically make use of the high force that can be generated by relatively compact hydraulic cylinders. Such devices are exemplified by U.S. Pat. No. 6,305,880 to Carter et al (“the Carter patent”). The Carter patent uses a pair of single acting hydraulic cylinders as the cable pulling device. The cylinders are are sufficiently small that the puller, along with a pulling frame, can be used by one or two operators and represents an improvement in the art over a winch based system.
The pulling device of the Carter patent however, nevertheless suffers from certain drawbacks. One drawback of the Carter device is that because the device uses single acting hydraulic cylinders, it has a relatively slow cycle time. Another drawback of the Carter device is that to prevent the cable from rebounding back into the pipe to be burst, the device requires additional hardware to hold the pulling cable stationary while the hydraulic cylinders make their return stroke.
Accordingly, there persists a need in the in art of trenchless pipe replacement for an improved cable pulling device that does not suffer the aforementioned drawbacks. Preferably, such a device would be smaller than the prior art winch-type devices, yet have a substantially faster cycle time than prior art hydraulic devices. A faster acting device would be more cost efficient for contractors and thus would reduce the time and cost of trenchless pipe replacement work.